I got to the point where I could generate objects similar to these:
These were actually the most acceptable out of several hundred bakes (only three front doors!) and I'm afraid getting the tool to generate acceptable output at all times is becoming way too time consuming. Instead, I'm simply going to hand-model and texture a series of basic solids and then pass sets of these through the CSG boolean tool:
There's no way to deform (like stretching them out without distorting them) these in any way yet (apart from orienting and scaling them), but it serves for now. Also tried to merge the whole scenery into a single solid (trees and foliage are still instanced) to trim away as much overdraw (especially the terrain is texfetch-expensive) as possible.